Anodes for mercury-cathode electrolytic cells

ABSTRACT

A process for forming on an anode an anodically-conducting oxidic coating having a porous outer layer of a chlorine resistant polymer bonded thereto, the process comprising coating the anode with the oxidic coating, applying to the said oxidic coating the polymer in admixture with a removable solid particulate material, bonding the polymer to the oxidic coating by the successive stages of drying, heating to melt the polymer and cooling, and subsequently removing the solid particular material from the coating. Suitable chlorine-resistant polymers for the porous outer layer of the anode include polyvinylidene fluoride and low molecular weight polytetrafluoroethylene (PTFE). The solid particulate materials include alkali metal chlorides, starch, cellulose, water-insoluble bases or carbonates.

United States Patent [1 1 Entwisle Nov. 25, 1975 1 ANODES FORMERCURY-CATI-IODE ELECTROLYTIC CELLS [75] Inventor: John HubertEntwisle, Runcorn,

England [73] Assignee: Imperial Chemical Industries Limited, London,England [22] Filed: Apr.'30, 1974 [21] Appl. No.: 465,558

[30] Foreign Application Priority Data May 16, 1973 United Kingdom23334/73 [52] U.S. Cl. 204/290 F; 136/120 FC; 427/115; 427/126; 427/375;427/376; 427/407', 427/409; 427/419 [51] Int. Cl. C23B ll/00; B32B 3/02[58] Field of Search 117/218, 63, 45, 212; 136/120 FC; 204/290 F;427/115, 126, 375, 379, 407, 419, 409

[56] References Cited UNITED STATES PATENTS 3,346,421 10/1967 Thompson136/120 FC 3,451,856 6/1969 Fraase et a1. 204/290 F 3,671,317 6/1972Rifkin 117/218 3,681,136 8/1972 Leonard et a1. 117/63 3,711,385 1/1973Beer 204/290 F 3/1974 Decraene 136/120 FC FOREIGN PATENTS ORAPPLICATIONS 875,718 7/1971 Canada 136/120 FC Primary Examine rMichaelF. Esposito Attorney, Agent, or FirmCushman, Darby & Cushman [57]ABSTRACT material, bonding the polymer to the oxidic coating by thesuccessive stages of drying, heating to melt the polymer and cooling,and subsequently removing the solid particular material from thecoating.

Suitable chlorine-resistant polymers for the porous outer layer of theanode include polyvinylidene fluoride and low molecular weightpolytetrafluoroethylene (PTFE).

The solid particulate materials include alkali metal chlorides, starch,cellulose, water-insoluble bases or carbonates.

16 Claims, No Drawings ANODES FOR MERCURY-CATHODE ELECTROLYTIC CELLS Thepresent invention relates to improvements in anodes for mercury-cathodeelectrolytic'cells. More particularly it relates to improvements incoated metal anodes used for the electrolysis of alkali-metal chloridesolutions in cells having flowing mercury cathodes.

In recent years attempts have been made to replace consumable graphiteanodes in mercury-cathode cells electrolysing alkali-metal chloridesolutions by dimensionally stable anodes made of titanium or a titaniumalloy carrying a thin anodically-conducting coating. The greatestsuccess has been achieved with oxidic coatings. These may, for example,consist of the electrically-conducting oxides of the platinum groupmetals, mixtures of these with the non-conducting oxide of one of thefilm-forming metals, titanium, tantalum, zirconium, niobium andtungsten, and/or with other refractory oxides such as tin dioxide,sometimes referred to a semiconducting mixtures, eg a mixture of theoxides of tin and antimony, to which a proportion of a platinum groupmetal oxide may be added to increase the electrocatalytic activity.

The aforesaid oxidic coatings have proved to be much more resistant todissolution and stripping from the titanium support by accidentalcontact with the mercury cathode in the working cell than are theearlier-proposed metallic coatings, eg platinum, iridium, rhodium andmixtures of these. Nevertheless even the oxide-coated anodes are capableof passing a large short-circuit current whenever they come into contactwith the cell cathode, and this current can lead to serious overheating,which can cause erosion of both the coating and the titanium support ifthe short-circuit condition is not quickly corrected. 'Since accidentalshort-circuiting of anodes cannot be entirely avoided, prevention ofshort-circuit damage is a serious problem.

The present invention provides a method of treating an anode of theoxide-coated titanium or titanium-alloy type so that the short-circuitcurrent between the anode and the mercury cathode at any given impressedvoltage is reduced without preventing passage of the electrolysingcurrent under normal operating conditions. The invention thus providesan anode of the oxide-coated titanium or titanium-alloy type which hasimproved resistance to damage by overheating under short-circuitconditions.

According to the present invention, therefore, there is provided aprocess for forming on an anode an anodically-conducting oxidic coatinghaving a porous outer layer of a chlorine resistant polymer bondedthereto, the process comprising coating the anode with the oxidiccoating, applying to the said oxidic coating the polymer in admixturewith a removable solid particulate material, bonding the polymer to theoxidic coating by the successive stages of drying, heating to melt thepolymer and cooling, and subsequently removing the solid particularmaterial from the coating.

From another aspect the present invention is a process for forming on ananode an anodically conducting oxidic coating having a porous outerlayer of chlorine I resistant polymer bonded thereto, the processcomprising coating the anode with the oxidic coating, coating the saidoxidic coating with a discontinuous layer of solid particulate materialso that a fraction of the oxidic coating is left uncovered betweenindividual particles of the particulate material, coating with thepolymer so as to fill the spaces between the particles, bonding thepolymer to the oxide by heating to melt the polymer followed by cooling,and subsequently removing the solid particulate material from thecoating. 7

Suitable chlorine-resistant polymers for the porou outer layer of theanode are polyvinylidene fluoride and low molecular weightpolytetrafluoroethylene (PTFE).

By a film-forming titanium alloy we mean an alloy sation propertiesthrough formation of a surface-film of oxide similar to those ofcommercially pure titanium. The anodically-conducting oxidic coating ofthe electrode may be any one of those known in the art which isresistant to electrochemical dissolution when connected anodically in anaqueous alkali-metal chloride electrolyte, particularly oxides of theplatinum group metals and the prior-art combinations of oxides listedhereinbefore. The preferred oxidic coatings are combinations of oxidesof one or more of the platinum group metals, platinum, iridium, osmium,ruthenium, rhodium and palladium, with the oxide of one of thefilmforming metals, titanium, tantalum, zirconium, niobium and tungsten,especially a combination of ruthenium dioxide and titanium dioxide.These combinations may be substantially homogeneous mixtures of whichthe platinum group metal oxide and the filmforming metal oxidecomponents have been laid down together on the titanium or titaniumalloy support member, or they may be coatings formed by depositingalternate layers of platinum group metal oxide and film-forming metaloxide on the support member, for instance as taught in British PatentSpecifications Nos. 1,206,863 and 1,294,373. With the latter method offorming the oxidic coating the homogeneity of the finished coating isless certain, but when the coating is made by firing layers of paintcompositions there is at least some interpenetration between the layersof the coating thereby deposited.

In order to produce the required porous outer layer ofchlorine-resistant organic polymer on the anode according to theinvention it is necessary to fix a coating of the polymer in the moltenstate to the oxide layer on the anode support member. This may be doneby preheating the oxide-coated support member to about 250C, sprayingthe molten polymer on to the oxide layer and then allowing the coatedelectrode to cool. Alternatively a coating of the polymer in a liquidvehicle may be applied to the oxide layer, the liquid vehicle than beingremoved by evaporation, the coated anode heated to about 250C to meltthe polymer coating and allowed to cool.

Thus an aqueous emulsion of low-molecular-weight PTFE may be brushed orsprayed on to the oxide coating, the coating then dried by heating inair at about C, after which the coated anode is heated at atemperaturenot higher than 280C for about 10 minutes to melt the polymer and isallowed to cool. The molecular weight of the PTFE must be restricted sothat it can be melted satisfactorily on the anode surface. A coating ofpolyvinylidene fluoride may suitably be applied in a similar mannerstarting with a solution or suspension of the polymer or alow-molecular-weight precursor in an organic solvent, eg in a mixture ofdimethyl phthalate and di-isopropyl ketone.

When the polymer coating is applied by one of the aforesaid procedures,the optimum thickness of the coating corresponds to a weight of aboutg/m of the coated surface. Substantially thicker layers may interfere toan unacceptable extent with passage of the normal electrolysing currentwhen the anode is in use, while layers less than about 3 g/m usuallyhave an adequate effect in limiting the short-circuit current in thecell. As aforesaid, the porosity of the polymer coating may be usefullyincreased by adding a removable particulate filler to an emulsion orsolution of the chlorineresistant organic polymer that is used forcoating the anode so that, after drying the coating, heating the driedcoating to melt the polymer and cooling, the particles of filler remainin the coating but are removed by water-washing or by reaction in theelectrolytic cell soon after the anode is put into service. Examples ofsuitable removable particulate fillers are alkali metal chlorides, egpotassium chloride, starch, cellulose or a water insoluble base orcarbonate, for example calcium carbonate. The particle size of thefiller is most suitably in the range 5 to 500 micron.

Another way of employing a removable particulate material to increasethe porosity of a polymer coating is first to coat the anode with adiscontinuous layer of the particulate material so that between theindividual particles a fraction of the anode surface remains uncovered,then to apply the polymer coating so as to fill the spaces between theparticles and then to bond the polymer coating to the anode surface byheating to melt the polymer. For example, a discontinuous layer ofpotassium chloride crystals may be formed on the anode surface bycoating the anode with a saturated aqueous solution of potassiumchloride and evaporating the aqueous medium. Then after applying andmelting the polymer coating the potassium chloride crystals may beremoved by water-washing before the anode is put to use or they may beallowed to dissolve in the electrolyte shen the anode is installed inthe electrolytic cell.

Coatings of increased porosity produced in accordance with either of thetwo preceding paragraphs have the advantage of being slightly thickerfor the same weight of polymer, although of reduced electricalresistance, and the total weight of polymer may also be increased withthe advantage of reducing the short-circuit current still furtherwithout interfering to an unacceptable extent with passage of the normalelectrolysing current through the anode.

The invention is further illustrated by the following Example:

EXAMPLE A strip of titanium 70 mm X 6 mm X 1 mm was etched in 10% oxalicacid solution, washed, dried and then provided with ananodically-conducting electrocatalytic coating consisting ofapproximately 40% Ru0 and 60% Ti0 by weight, by applying to it eightlayers of a paint composition prepared by dissolving rutheniumtrichloride and tetrabutyl orthotitanate in npentanol, each paint layerbeing dried in air at 180C and then heated in air for minutes at 450C.The coated titanium strip was then dipped into a saturated aqueoussolution of potassium chloride and withdrawn. The layer of solutionremaining on the coated strip was allowed to dry out leaving a film ofsmall potassium chloride crystals covering about 80% of the coatedsurface. The cracks between the crystals were filled by brushing on asuspension of low-molecular-weight polyvinylidene fluoride in an organicsolvent (sold as Kynar 500). The coated strip was then heated at 250Cfor about 4 minutes to cause further polymerisation of thepolyvinylidene fluoride and bonding of the polymer tothe coating of RuO/TiO between the potassium chloride crystals. The coated strip was thenwashed in water to remove the potassium chloride crystals and wasafterwards used as an anode to electrolyse sodium chloride brine in alaboratory-scale mercurycathode cell.

The anode strip was suspended in the cell with its faces in a verticalplane and its longest axis horizontal. When a short circuit was causedby raising the level of the mercury cathode until the bottom 4 mm of thestrip were immersed in the mercury, a short-circuit current of only 3amp/cm length of the anode strip flowed between the anode and cathode.This may be compared with a short-circuit current of 10 to 15 amp/cmlength observed under the same operating conditions with a coatedtitanium anode strip of the same size carrying the same electrocatalyticcoating of RuO /TiO but without the superimposed layer of polyvinylidenefluoride.

What I claim is:

1. A process for forming improved anodes for mercury-cathodeelectrolytic cells, by forming on an anode an anodically-conductingoxidic coating having a porous outer layer of a chlorine resistantpolymer bonded thereto, said polymer being polyvinylidene fluoride orlow molecular weight polytetrafluoroethylene, the process comprisingcoating the anode with the oxidic coating, wherein the oxidic coatingcomprises a platinum group metal oxide; applying to said oxidic coatingthe polymer in admixture with a removable solid particulate material,wherein said solid particulate material is an alkali metal chloride,starch, cellulose, a water insoluble base or carbonate thereof; bondingthe polymer to the oxidic coating by successive stages of drying byheating in air at about C, heating to melt the polymer at temperaturesof up to 280C, and cooling; and subsequently removing the solidparticulate material from the coating by dissolving the particulatematerial by dissolving in water or dilute acid, whereby the shortcircuitcurrent between the anode and the mercury cathode at any given impressedvoltage is reduced without preventing passage of the electrolyzingcurrent under normal operating conditions.

2. A process as claimed in claim 1 wherein the solid particulatematerial is an alkali metal chloride, starch or cellulose.

3. A process as claimed in claim 1 wherein the solid particulatematerial is removed by reaction in the electrolytic cell soon after theanode is put on service.

4. A process as claimed in claim 1 wherein the solid particulatematerial is a water-insoluble inorganic base or carbonate.

5. A process as claimed in claim 4 wherein the solid particulatematerial is calcium carbonate.

6. A process as claimed in claim 4 the solid particulate material isremoved by dissolving in acid or by reaction in the electrolytic cellsoon after the anode is put in service.

7. A process as claimed in claim 1 wherein the oxidic coating comprisesa platinum group metal or oxide thereof in admixture with an oxide of afilm forming metal.

8. A process as claimed in claim 7 wherein the oxidic coating comprisesruthenium dioxide and titanium dioxide.

9. The process of claim 1, wherein the thickness of the polymer coatingis up to 5 glm 10. An anode for a mercury cathode electrolytic cell,comprising a support member made of titanium or a film-forming titaniumalloy and bonded thereto an anodically-conducting oxidic coating, saidcoating being a platinum group metal or an oxide thereof, having aporous outer layer of a chlorine-resistant organic polymer bondedthereto, said polymer being polyvinylidene fluoride or low molecularweight polytetrafluoroethylene, and wherein the anode is coated by theprocess claimed in claim 1.

11. A process for producing an improved anode for mercury-cathodeelectrolytic cells, by forming on an anode anodically-conducting oxidiccoating having a porous outer layer of a chlorine resistant polymerbonded thereto, said polymer being polyvinylidene fluoride or lowmolecular weight polytetrafluoroethylene, the process comprising coatingthe anode with the oxidic coating, wherein the oxidic coating comprisesa platinum group metal oxide; coating the said oxidic coating with adiscontinuous layer of solid particulate material, wherein said solidparticulate material is an alkali metal chloride, starch, cellulose, awater insoluble base or carbonate thereof, so that a fraction of theoxidic coating is left uncovered between individual particles of theparticulate material; coating with the polymer so as to till the spacesbetween the particles; bonding the polymer to the oxide by heating attemperatures up to 280C to melt the polymer, followed by cooling; andsubsequently removing the solid particulate material from the coating bydissolving in water or dilute acid, whereby the short-circuit currentbetween the anode and the mercury cathode at any given impressed 6voltage is reduced without preventing passage of the electrolyzingcurrent under normal operating conditions.

12. A process as claimed in claim 11 wherein the anodically-conductingoxidic coating comprises a platinum group metal oxide.

13. The process of claim 11, wherein the thickness of the polymercoating is up to 5 g/m 14. A process as claimed in claim 11, wherein theoxidic coating comprises ruthenium dioxide and titanium dioxide.

15. A process for forming on an anode an anodically conducting oxidiccoating have a porous outer layer of a chlorine-resistant polymer bondedthereto said polymer being polyvinylidene fluoride or low molecularweight polytetrafluoroethylene, the process comprising coating the anodewith the oxidic coating said oxidic coating being a platinum group metaloxide, heating the coated anode to about 250C, spraying molten polymeron to the oxide layer and allowing the coated anode to cool.

16. A process for forming on an anode an anodically conducting oxidiccoating having a porous outer layer of a chlorine-resistant polymerbonded thereto said 7 polymer being polyvinylidene fluoride or lowmolecular weight polytetrafluoroethylene, the process comprising coatingthe anode with the oxidic coating said oxidic coating being a platinumgroup metal oxide, applyingto thesaid'coating the polymer in a liquidvehicle, removing the liquid vehicle by evaporation, and

, heating the coated anode to about 250C to melt the polymer coating,and allowing the coated anode to cool.

1. A PROCESS FOR FORMING IMPROVED ANODES FOR MERCURYCATHODE ELECTROLYTIC CELLS, BY FORMING ON AN ANODE AN ANODICALL-CONDUCTING OXIDIC COATING HAVING A POROUS OUTER LAYER OF A CHLORINE RESISTANT POLYMER BOND THERETO, SAID POLYMER BEING POLYVINYLIDENE FLUORIDE OR LOW MOLECULAR WEIGHT POLYTETREFLUOROETHYLENE, THE PROCESS COMPRISING COATING THE ANODE WITH THE OXIDIC COATING, WHEREIN THE OXIDIC COATING COMPRISES A PLATINUM GTOUP METAL OXIDE, APPLYING TO SAID OXIDI COATING THE POLYMER IN ADMIXTURE WITH A REMOVABLE SOLID PARTICULATE MATERIAL, WHEREIN SAID SOLID PARTICULATE MATERIAL IS AN ALKALI METAL CHLORIDE, STARCH, CELLULOSE, A WATER INSOLUBLE BASE OR CARBONATE THEREOF, BONDING THE POLYMER TO THE OXIDIC COATING BY SUCCESSIVE STAGES OF DRYING BY HEATING IN AIR AT ABOUT 160*C, HEATING TO MELT THE POLYMER AT TEMPERATURES OF UP TO 280*C, AND COOLING, AND SUBSEQUENTLY REMOVING THE SOLID PARTICULATE MATERIAL FROM THE COATING BY DISSOLVING THE PARTICULATE MATERIAL BY DISSOLVING IN WATER OR DILUTE ACID, WHEREBY THE SHORTCIRCUIT CURRENT BETWEEN THE ANODE AND THE MERCURY CATHODE AT ANY GIVEN IMPRESSED VOLTAGE IS REDUCED WITHOUT PREVENTING PASSAGE OF THE ELECTROLYZING CURRENT UNDER NORMAL OPERATING CONDITIONS.
 2. A process as claimed in claim 1 wherein the solid particulate material is an alkali metal chloride, starch or cellulose.
 3. A process as claimed in claim 1 wherein the solid particulate material is removed by reaction in the electrolytic cell soon after the anode is put on service.
 4. A process as claimed in claim 1 wherein the solid particulate material is a water-insoluble inorganic base or carbonate.
 5. A process as claimed in claim 4 wherein the solid particulate material is calcium carbonate.
 6. A process as claimed in claim 4 the solid particulate material is removed by dissolving in acid or by reaction in the electrolytic cell soon after the anode is put in service.
 7. A process as claimed in claim 1 wherein the oxidic coating comprises a platinum group metal or oxide thereof in admixture with an oxide of a film forming metal.
 8. A process as claimed in claim 7 wherein the oxidic coating comprises ruthenium dioxide and titanium dioxide.
 9. The process of claim 1, wherein the thickness of the polymer coating is up to 5 g/m2.
 10. An anode for a mercury cathode electrolytic cell, comprising a support member made of titanium or a film-forming titanium alloy and bonded thereto an anodically-conducting oxidic coating, said coating being a platinum group metal or an oxide thereof, having a porous outer layer of a chlorine-resistant organic polymer bonded thereto, said polymer being polyvinylidene fluoride or low molecular weight Polytetrafluoroethylene, and wherein the anode is coated by the process claimed in claim
 1. 11. A process for producing an improved anode for mercury-cathode electrolytic cells, by forming on an anode anodically-conducting oxidic coating having a porous outer layer of a chlorine resistant polymer bonded thereto, said polymer being polyvinylidene fluoride or low molecular weight polytetrafluoroethylene, the process comprising coating the anode with the oxidic coating, wherein the oxidic coating comprises a platinum group metal oxide; coating the said oxidic coating with a discontinuous layer of solid particulate material, wherein said solid particulate material is an alkali metal chloride, starch, cellulose, a water insoluble base or carbonate thereof, so that a fraction of the oxidic coating is left uncovered between individual particles of the particulate material; coating with the polymer so as to fill the spaces between the particles; bonding the polymer to the oxide by heating at temperatures up to 280*C to melt the polymer, followed by cooling; and subsequently removing the solid particulate material from the coating by dissolving in water or dilute acid, whereby the short-circuit current between the anode and the mercury cathode at any given impressed voltage is reduced without preventing passage of the electrolyzing current under normal operating conditions.
 12. A process as claimed in claim 11 wherein the anodically-conducting oxidic coating comprises a platinum group metal oxide.
 13. The process of claim 11, wherein the thickness of the polymer coating is up to 5 g/m2.
 14. A process as claimed in claim 11, wherein the oxidic coating comprises ruthenium dioxide and titanium dioxide.
 15. A process for forming on an anode an anodically conducting oxidic coating have a porous outer layer of a chlorine-resistant polymer bonded thereto said polymer being polyvinylidene fluoride or low molecular weight polytetrafluoroethylene, the process comprising coating the anode with the oxidic coating said oxidic coating being a platinum group metal oxide, heating the coated anode to about 250*C, spraying molten polymer on to the oxide layer and allowing the coated anode to cool.
 16. A process for forming on an anode an anodically conducting oxidic coating having a porous outer layer of a chlorine-resistant polymer bonded thereto said polymer being polyvinylidene fluoride or low molecular weight polytetrafluoroethylene, the process comprising coating the anode with the oxidic coating said oxidic coating being a platinum group metal oxide, applying to the said coating the polymer in a liquid vehicle, removing the liquid vehicle by evaporation, and heating the coated anode to about 250*C to melt the polymer coating, and allowing the coated anode to cool. 